Madison Avenue's center of gravity is shifting. Omnicom is now officially the world's largest ad agency holding company, thanks to its $9 billion acquisition of Interpublic Group, which closed Friday.The combination brings together creative and media agencies, health marketing specialists, and production studios. They are all set to be underpinned by data from its Acxiom data-management company and Omni, which the company describes as its advanced intelligence platform. The agency mega-merger is also set ...